Pellet burning stoves of the type disclosed in U.S. Pat. Nos. 4,619,209, 4,669,396, and 4,779,554 have become very popular in recent years due to their low emission levels, high combustion efficiency, and ease of operation. Such stoves typically comprise a combustion chamber, a burn pot coupled with the combustion chamber for supporting pellets during the combustion process, a hopper for storing pellets, and an auger assembly for transporting pellets from the hopper to the burn pot. The burn pot of pellet burning stoves of the type disclosed in the patents identified above typically comprises a plurality of holes extending through the sidewall of the burn pot through which combustion air is introduced to the pot.
In an attempt to improve the combustion efficiency and reduce the quantity of harmful gases emitted from pellet stoves, fan systems have been added to such stoves to force combustion air into the burn pot or to draw exhaust gases out of the combustion chamber. Indeed, it is believed that all pellet stoves currently marketed in the United States which are designed to emit exhaust gases having a carbon monoxide concentration, by volume, of less than 0.04% utilize a fan system for forcing air into, or drawing gases out of, the stove. Because the speed of the fan must be automatically varied based on the desired heat output, the actual burn rate, and other factors to ensure clean burning and high combustion efficiency, a microprocessor-controlled fan adjustment system is required. For instance, the clean burning commercial embodiment of the pellet stove disclosed in U.S. Pat. No. 4,779,544, i.e., the embodiment designed to emit exhaust gases having CO levels of less than 0.04%, uses an exhaust fan to extract exhaust gases from the stove. Although the '544 patent is silent regarding the manner in which exhaust gases are withdrawn from the stove, the construction of the stove is such that an exhaust fan is believed to be required to achieve clean burning, as evidenced by the use of an exhaust fan on the commercial embodiment of the stove of the '544 patent (the Welenco Pellet Heater manufactured by Welenco Mfg. Inc., Lewiston, Id.).
The use of one or more fans and an associated control system typically adds about $200 to $300 to the retail cost of such a stove. Additionally, although the control system can be shielded to some extent from the high temperatures adjacent the stove, in practice the service life of the printed circuit board and other components of the fan control system is often reduced to an unacceptably short period as a consequence of the high temperature environment in which the control system must operate.
To ensure a pellet stove burns as cleanly as possible, the quantity of combustion air introduced into the burn pot through the apertures in the sidewall thereof must be carefully controlled. In pellet stoves designed to emit exhaust gases having CO concentrations of less than 0.04%, delivery of the proper quantity of combustion air is typically achieved by appropriate operation of the fan system for introducing combustion air into or withdrawing exhaust gases from the stove. Because delivery of a proper quantity of combustion air is virtually ensured if appropriately sized fans are used and if the microprocessor of the fan control system is correctly programmed, the size, number, and placement of the apertures in the sidewall of the burn pot through which combustion air is introduced are not critical to the clean burning operation of the stove. As a consequence, it is believed that little research has been conducted regarding the size, number, and placement of the combustion air intake apertures in the burn pots of pellet burning stoves required to achieve optimal combustion efficiency and minimal emissions of harmful exhaust gases.
Recently, carbon monoxide concentration in stove exhaust gases has become regarded by many as the preferred indicator of overall cleanliness of stove exhaust, with CO levels below 0.04% being achieved only by the cleaner-burning pellet stoves. A carbon monoxide concentration of 0.04% correlates closely with a particulate count of about 7 grams per hour, the current EPA standard for wood stoves not equipped with a catalytic converter. Particulate count is the older, and many feel the less accurate, technique for determining combustion efficiency of a stove.
One of the problems associated with some pellet burning stoves is "back burn." Back burn occurs when pellets are fed into the burn pot faster than they are consumed by the fire. This causes an accumulation of unburned pellets in the burn pot and subsequently an overflow of the pot. If not properly designed, the overflow causes the pellets to back up in the feed chute, thus allowing the fire to follow the pellets up the feed chute and possibly into the pellet storage area.